Method for improving areal sweep efficiency in solvent recovery processes



United States Patent 3,245,467 METHOD FOR IMPROVING AREAL SWEEP EFFI- CIENCY IN SOLVENT RECOVERY PROCESSES Richard A. Fitch, Tulsa, Okla, assiguor to Pan American Petroleum Corporation, Tulsa, Okla, a corporation of Delaware No Drawing. Filed Dec. 20, 1962, Ser. No. 245,987 Claims. (Cl. 166-9) The present invention relates to the recovery of hydrocarbons from an underground reservoir thereof. More particularly, it is concerned with a method for improving the efficiency of oil recovery by fluid drive techniques involving the use of a hydrocarbon solvent.

Briefly stated, my invention comprises placing an initial water bank ahead of the solvent employed in a secondary recovery process for the purpose of stabilizing the solvent bank employed in said process.

Miscible fluid and other types of drives involving the use of a leading bank of hydrocarbon solvent have long been regarded as the simplest methods to obtain displacement efliciencies significantly higher than those characteristic of conventional water flooding. One of the drawbacks of such procedures, however, has been the adverse mobility ratios typical of miscible fluid drives which result in low areal sweep efliciencies. Generally speaking, the lower areal sweep efficiency, larger investment and larger operating costs of the various miscible fluid drive processes have made them less attractive in field applications than ordinary water flooding.

It is known that the areal sweep efliciency at breakthrough obtained in a given type of displacement can be correlated with the mobility ratio, and numerous suggestions have been made for the purpose of improving such ratio which, in turn, meant an improvement in the areal sweep efficiency. One possibility was the simultaneous injection of gas and water as the driving phase behind a miscible zone. Also, the simultaneous injection of solvent and water behind the miscible zone has been suggested to improve areal sweep efficiency. Neither of these methods, however, has met with unqualified success.

One of the chief difliculties in the use of the miscible fluid technique results from the tendency of the solvent bank to extend itself into long streaks or fingers near the injection well bore. This tendency is accentuated as the bank of solvent travels through the formation toward the producing well and, accordingly, directly influences the amount of recovery at breakthrough of the solvent.

Accordingly, it is an object of my invention to provide means for improving displacement of fluids through a reservoir by stabilization of the solvent bank employed in a miscible fluid type of drive. It is another object of my invention to provide a means for stabilizing said bank where the latter is being forced through a reservoir by means of a drive agent that may consist of a single fluid or a gas and a liquid, and wherein such gas and liquid are being injected alternately or simultaneously into the reservoir. It is still another object of my invention to provide a method for increasing oil recovery at breakthrough of solvent into the producing well or wells. It is a further object of my invention to improve oil recovery in miscible fluid drive procedures, particularly where such methods as used in reservoirs that are highly stratified or have large variations in areal permeability.

I have now discovered that increased oil recovery through the use of a miscible fluid drive, for example, can be effected by injecting a water bank into the formation in the vicinity of the injection well prior to the introduction of the solvent bank which, in turn mayvbadriven by ggszor by alternate or simultaneous injection of water and gas or solvent slugs. This initially injected water bank Patented Apr. 12, 1966 inhibits the tendency of the solvent (oil-miscible phase) to finger and produce a highly unstable or nonuniform front, particularly near the injection well bore where this tendency is most detrimental to a good areal sweep efliciency.

When a less mobile Water bank is injected initially, a substantially uniform front is formed. Subsequent injection of the miscible phase (oil solvent) causes-finge-r-like streaks of solvent to extend throughout the water bank. However, when the immiscible, more mobile solvent phase has completely penetrated the Water bank and has moved ahead, it tends to distribute itself more uniformly in front of said bank. The drive agent made up-for example-of alternate slugs of solvent and water, or solvent and gas, subsequently injected is thus led by a more uniformly distributed miscible zone. This, in turn, results in an increased recovery efficiency at breakthrough due to an improved areal sweep efficiency.

A secondary benefit from the method of my invention results in those reservoirs having a large variation in horizontal permeability throughout the pay section. This is true because the initial water bank tends to penetrate farthest into sections with highest permeability. During subsequent injection of the miscible phase, these sections do not appear in the general flooding pattern as nonuniform to the miscible phase owing to the relatively higher penetration of water into said sections.

By applying my invention to a solvent recovery method of the type referred to above, solvent breakthrough can be delayed, resulting in a very substantial oil recovery at this stage of the process over that experienced when using conventional methods. Moreover, the ultimate recovery, i.e., the point at which the water-oil ratios become uneconomic, obtainable by the use of my invention is substantially greater than is possible by said conventional methods. In this connection, it is desirable to delay breakthrough of solvent and drive fluid because of: (1) the difficulty in handling larger volumes of produced gas (where gas is used, either alone or in combination with another fluid as the drive agent) and possible restrictions on oil production at high gas-oil ratios; (2) the loss of gas and solvent from the reservoir which otherwise might further sustain the miscible front and ultimately be trapped in pore spaces that ordinarily would be occupied by oil.

In my invention I have taught that the increase in re covery on solvent breakthrough, under the conditions I have investigated, amounts, on the average, to about 10 percent of the hydrocarbon pore volume (hereinafter referred to as HCPV) when an initial water bank is employed. Thus, in solvent flooding operations where no initial water bank was used, the average oil recovery at solvent breakthrough was found to be 31.6 percent of the HCPV, whereas the average recovery at solvent breakthrough when my invention was used amounted to 41.5 percent of the HCPV. In these cases, the solvent bank was forced through the reservoir by alternately injected solvent and water plugs. In two additional tests, using initial water banks, oil recoveries of 42.7 and 56.0 percent were experienced for an alternate solvent-water flood. Breakthrough recovery for alternate use of solvent-water slugs where solvent was injected initially, i.e., not preceded by a water bank, was found to be the same as for an ordinary solvent drive which gave 30 percent oil recovery at breakthrough. Thus, it is seen that through use of an initial water bank the recovery at breakthrough performance can be improved by values of from 12.7 to 26.0 percent of the HCPV.

The size of the initial water bank may vary widely and still be effective in producing the phenomenon taught therein. In most instances, the water employed in the initial bank may vary from about 1 to about 10 percent 3 of the HCPV, typically from about 2 to about percent.

The amount of solvent needed, likewise, may vary. However, in general, the quantity used in carrying out the process of my invention may be the same as that employed in a miscible fluid drive, as now known in the art. In case the drive agent comprises alternate slugs of solvent and water, the solvent-water ratio may vary from about 1:5 to about 2:1. Solvents used in the miscible drive method generally vary in quantity from about 1 to about 15 percent of the HCPV, typically from about 5 to about percent. The solvents employed may be any of several wellknown materials that are substantially immiscible with water, such as-for examplepropane, butane, kerosene, gasoline, LPG, and the like. Reservoir conditions, of course, should be such that the solvents is maintained in the liquid phase while the drive agent may or may not be in the same phase as is described in detail in copending application U.S. Serial No. 293,544, filed June 14, 1952, by R. A. Morse.

My invention is further illustrated by reference to the series of tests described below. In this work, two wells extending into the oil-bearing sand were employed. With one group of tests, an initial water bank was injected into the pay, followed by a solvent bank which, in turn, was followed by alternate injection of a solvent and water. In another series of trials, the conditions were the same as those just mentioned except the initial water bank was omitted. A miscible fluid drive and an ordinary waterflood were also carried out, as will be seen from the table below. The tests reported herein were all conducted in models representing one-eighth of a five-spot pattern. These models were prepared as follows: A block of bond ed silica filter media, referred to in the trade as Filtros, Was cut to the desired dimension. Next, a thin coating Armstrong X-85 epoxy resin was applied to the rock sealing all surfaces. Penetration was held to a minimum because this is one of the most viscous resins available. After the first coat of resin had cured, a second, thicker coat was applied to provide strength and to cover any small areas not sealed completely by the first coat. Well bores were cut into the model by cutting through the layer of resin into the rock phase With a small saw blade. Plastic cylinders which were milled and tapped to provide for fittings were then attached to the model over the well In Tests 1 to 3, the solvent consisted of 62 percent trichloroethane and 38 percent heptane; the water phase was a :50 mixture of glycerine and distilled water, and the oil phase was made up of 45 percent mineral oil and percent trichloroethane. In Tests 4 to 9, inclusive, the model was treated with n-Octylamine to make it oil-wet. However, for the tests the fluid system was inverted (water phase represented oil and vice versa) and the data are reported such that they represent a water-wet system. This procedure was used to insure that wettability would remain consistent during the tests. The solvent phase in these tests consisted of distilled water, the water phase was composed of 67 percent mineral oil and 33 percent trichloroethane, and the oil phase contained 57 percent glycerine and 43 percent water. Where alternate injection of water and solvent was used, the duration of a specific pair of injection cycles (water and solvent) corresponded approximately in minutes to the water-solvent ratio shown in the table. In other words, an injection ratio of 5 parts water to 1 part solvent would require about 5 minutes for water injection and 1 minute for injection of solvent.

In these tests, the model was evacuated and saturated with the in-place fluid. The driving fluid was injected into the desired well by a Zenith constant rate pump. The produced fluid was collected incrementally and analyzed for composition. After completion of the tests, the inplace fluid was injected until the model was again saturated with this phase.

In those tests where immiscible fluids were used, the following steps were taken: The model was initially saturated with the wetting phase, water; the desired in-place oil phase was then injected until any remaining water was immobile. Resaturation after a test in these models usually was accomplished by again injecting the oil phase unitl all mobile water and any other fluids present had been removed.

In the alternate injection tests, two Zenith pumps were used alternately for fluid injection at a constant rate. The production here also was usually collected and analyzed incrementally. Differential pressure across the model was recorded automatically during all tests and furnished the most exact indication of breakthrough.

bores to permit flow lines and valves to be installed. 45 The results obtained are shown in the table below.

Table Oil Phase Water Phase Solvent Phase Mobility Ratio Mfibllity atio Test Type of test Calculated Number Vis- Density Vis- Denslty, V1S- Density, Water Solvent Alternate cosity, g./ee. cosity, gJcc. cosity g./ec. Flood Drive Solventp- P- P- Water Alternate Solvent-Water (Initial Water 4. 84 1. 075 6. 36 1. 136 572 1. 084 408 8. 46 447 Bank Misciblg Drive 4. 84 l. 07 5 6. 36 1. 136 572 1. 084 426 8. 46 Alternate Solvent-Water (Initial Water 4. 84 1. 075 6. 36 1. 136 572 1. 084 426 8. 46 473 B nk Misgiblg Drive 9. 1 1. 157 9. 8 1. 014 89 1. 000 492 10. 2 Alternate S01vent-Water 9. 1 1. 157 10. 0 1. 014 89 1. 000 485 10. 2 542 dn 8. 8 1.157 10. 0 1.012 89 1.000 461 9. 9 531 Water Flood 8. l. 157 10. 3 1. 008 89 l. 000 441 9. 7 Alternate Solvent-Water (Initial Water 8. 65 1. 157 10. 6 1. 004 89 1. 000 433 9. 7 502 Bank 9 rln 8.65 1.157 10.9 1. 000 .89 1.000 422 9.7 .545

Alternate Slug Size Recovery Perme- Connate Injection Initial Percent HCPV Percent HCPV Test ability, Rate, Water V Ratjigml Blznk Sizte, Flllltigllllillllllal D s cc. D111. Saturation, ater 0 v. ereen a1 Number arcy 1 Percent HCPV Water Solvent At or A1; WBT

9 224 5/1 9. 00 1. 9 244 9 244 4/1 7. 2 1. 80 12 24 12 24 4/1 4. 87 1. 22 12 232 3/1 3. 65 1. 22 12 23 55. 8 12 23 3/1 7. 3 Water 3. 65 1. 22 43. 9 5, 3 12 232 3/2 7. 3 do 3. 65 2. 44 39.1 79. 5

The advantages resulting from the use of an initial water bank ahead of a solvent bank in a miscible fluid or solvent drive are apparent from the above table. Thus, in Tests Nos. 1, 3, 8 and 9, higher oil recoveries at solvent and at water breakthrough were secured than in any of the other trials employing conventional methods without the use of an initial water bank. For example, a comparison of the results in Tests Nos. 1 and 2 shows that an increase of almost 100 percent in oil recovery at solvent breakthrough was experienced when an initial water bank was employed over the recovery obtained without the use of such bank. There is no reason to believe that such advantage would not carry through and be shown to about the same extent in terms of ultimate oil recovery.

It will be apparent that the principle of my invention is applicable to a wide variety of solvent oil recovery systems and that the medium driving the solvent bank through the reservoir has very little, if any, effect on the influence of the initial water bank and the tendency of the latter to stabilize or deaccentuate the streaking or fingering tendency of the solvent. Thus, such drive agents may be gas, mixtures of gas and water, or aqueous solutions of materials substantially immiscible with oil. Accordingly, the expression drive agent is intended to include such fluid systems or mixtures thereof. Also, as used in the present description and claims, the term Water is intended to include aqueous systems having essentially the same characteristics as water under the conditions of use contemplated herein.

I claim:

1. In a method of recovering crude oil from a hydrocarbon-containing, Water-wet reservoir having a producing nwcllwandaaninjection Well extending into said reservoir, the improvement which comprises first injecting water into said reservoir through said injection well in an amount suflicient to form a bank or front of liquid water of relatively low mobility in the neighborhood of said injection well, the amount of injected water not exceeding 10 percent of the hydrocarbon pore volume of said reservoir," thereafter injecting into said reservoir via said injection well a normally gaseous hydrocarbon solvent for said oil in an amount sufiicient and under conditions adequate to form a liquid solvent bank contiguous to said water bank,

next injecting into said reservoir via said injection well a fluid drive agent at a pressure sufficient to cause ...fil g:l li @l ..19m...%Qd.9bltlt throughout said water bank and to thereafter emerge Miriam .l Y .%l. -bI1 $.lkihlldlfi p f 3 Proliing well whereby said solvent tends to distribute itself in a more uniform bank at the leading edge of the water bank, continuing the introduction of said agent via said injection well under the aforesaid conditions to drive said solvent #bank through said reservoir and toward said producing well, and withdrawing crude oil from said producing well. 2. The process of claim 1 in which said drive agent is composed of alternate slugs of gas and water.

3. The process of claim 2 in which the ratio of water to gas employed as the drive agent ranges from about 5:1 to about 1:2.

4. The process of claim 1 in which said drivemagent I is composed of alternate slugs of a yglfgcarbop solyeggjl and water.

5. The process of claim 1 in which said drive agent is water.

6. The process of claim 1 in which said drive agent is gas.

7. The process of claim 6 in which said gas is natural gas.

8. The process of claim 6 in which said natural gas is injected at a pressure sufliciently high to form a single phase liquid transition zone between said solvent bank and said natural gas.

9. The method of claim 1 in which the solvent is a hydrocarbon having from about 3 to about 5 carbon atoms. 4

10. The process of claim 1 in which the solvent is LPG.

References Cited by the Examiner UNITED STATES PATENTS 2,798,556 7/ 1957 Binder 166-9 2,927,637 3/1960 Draper 166-9 3,074,481 1/ 1963 Habermann 166-9 3,100,524 8/1963 Beeson 166-9 3,170,513 2/1965 Dew et al. 166-9 CHARLES E. OCONNELL, Primary Examiner.

C. H. GOLD, T. A. ZALENSKI, Assistant Examiners.

bank to extend 

1. IN A METHOD OF RECOVERING CRUDE OIL FROM A HYDROCARBON-CONTAINING WATER-WET RESERVOIR HAVING A PRODUCING WELL AND AN INJECTION WELL EXTENDING INTO SAID RESERVOIR, THE IMPROVEMENT WHICH COMPRISES FIRST INJECTING WATER INTO SAID RESERVOIR THROUGH SAID INJECTION WELL IN AN AMOUNT SUFFICIENT TO FORM A BANK OR FRONT OF LIQUID WATER OF RELATIVELY LOW MOBILITY IN THE NIEGHBORHOOD OF SAID INJECTION WELL, THE AMOUNT OF INJECTED WATER NOT EXCEEDING 10 PERCENT OF THE HYDROCARBON PORE VOLUME OF SAID RESERVOIR, THEREAFTER INJECTING INTO SAID RESERVOIR VIA SAID INJECTION WELL A NORMALLY GASEOUS HYDROCARBON SOLVENT FOR SAID OIL IN AN AMOUNT SUFFICIENT AND UNDER CONDITIONS ADEQUATE TO FORM A LIQUID SOLVENT BANK CONTIGUOUS TO SAID WATER BANK, NEXT INJECTING INTO SAID RESERVOIR VIA SAID INJECTION WELL A FLUID DRIVE AGENT AT A PRESSURE SUFFICIENT TO CAUSE FINGER-LIKE STREAKS FROM SAID SOLVENT BANK TO EXTEND THROUGHOUT SAID WATER BANK AND TO THERAFTER EMERGE FROM SAID WATER BANK IN THE DIRECTION OF SAID PRODUCING WELL WHEREBY SAID SOLVENT TENDS TO DISTRIBUTE ITSELF IN A MORE UNIFORM BANK AT THE LEADING EDGE OF THE WATER BANK, CONTINUING THE INTRODUCTION OF SAID AGENT VIA SAID INJECTION WELL UNDER THE AFORESAID CONDITIONS TO DRIVE SAID SOLVENT BANK THROUGH SAID RESERVOIR AND TOWARD SAID PRODUCING WELL, AND WITHDRAWING CRUDE OIL FROM SAID PRODUCING WELL. 